High-frequency attenuator and divider circuits



- Oct. 15, 1946.

R. e. CLAPP HIGH FREQUENCY ATTENUATOR AN D DIVIDER CIRCUITS Filed May 13, 1943 Oct. '15, i946. v R. G. CLAPP 7 I 2 ,4

HIGH FREQUENCY ATTENUATOR AND liIYIDER CIRCUITS F'il l May 13, 1943 2 Sheets-Sheet 2 h Patented Oct. 15, 1946 UNITED STATES PATENT OFFICE HIGH-FREQUENCY ATTENUATOR AND DIVIDER CIRCUITS vania Application May 13, 1943, Serial No. 486,877

Claims. 1

This invention relates tohigh frequency control circuits, and more particularly to high frequency attenuator andhigh frequency divider circuits and structures.

Attenuator and divider structures and circuits adapted for use at audio frequencies and at low radio frequencies are well known in the art, and there are numerous conventional and satisfactory devices available for such applications. At very high frequencies, however, say in the tens or hundreds of megacycles, these conventional devices are of no utility.

Accordingly, it is a principal object of this invention to provide an attenuator or divider circuit capable of performing satisfactorily at very high frequencies.

It is another object of this invention to provide a simple attenuator or divider structure for use at very high frequencies.

It is a further object of this invention to provide a variable voltage divider device capable of dividing a high frequency signal between two load circuits in any desired ratio.

It is another object of the invention to provide an attenuator for dividing a high frequency signal between two load circuits in any desired proportion with minimum reaction on the signal source.

These and other objects of the invention will be apparent from the following description of the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram of a high frequency attenuator adapted to divide an input signal between two load circuits;

Fig. 2 is a sectional view of an attenuator and divider device constructed in accordance with the teachings of the present invention;

Fig. 3 is a sectional View taken along line 3-3 of Fig. 2; and

Figs. 4, 5, and 6 are explanatory illustrations showing, in polar coordinates, typical attenuation curves which may be provided by the apparatus of Figs. 1 to 3.

Reference may now be had to Fig. 1, wherein E is a source of high frequency signals, and T1 L2 is preferably a one-turn loop (see Fig. 3) of which only the middle portion is magnetically coupled to L1 through an opening 2 in the shield structure 3. Because of the presence of the shield there is no appreciable electrostatic coupling between the primary winding L1 and the secondary winding L2. The grounded contact member I may be arranged, by any suitable mechanical means, to slide on the secondary inductance L2, so that it can be positioned to ground any desired point thereon.

By means of the arrangement described, the middle portion of L2 serves somewhat as a variable auto-transformer, while the two ends thereof function as variable series inductance elements in the attenuator. The shunt resistor R1, which is connected across the secondary inductance L2, may have a relatively low resistance, for example 200 ohms. The resistors R3 are terminating resistors for the coaxial transmission lines T1 and T2 and may have conventional resistance values. The resistance of the line coupling resistors R2 should be high in comparison with the resistance of R1 and R3 and may be, for example, of the order of a thousand ohms. The resistors R2 serve to isolate the transmission lines from the attenuator, and help to keep the load on the inductance L2 constant, regardless of the position of the contact member 5. The switch S of Fig. l constitutes a refinement, and for the moment it will be assumed to be absent.

A satisfactory mechanical structure for the device illustrated schematically in Fig. 1 is illustrated in Figs. 2 and 3, wherein like reference characters designate corresponding elements in the circuit diagram of Fig. 1. In the embodiment illustrated, the single-turn inductance L2 is insulatingly supported from the base portion of the shield 3 by means of the stand-cit insulators 4 and 5.

The shield structure 3, in addition to preventing electrostatic coupling between the inductances L1 and L2, also shields the two lines T1 and T2 from both of the inductances. Preferably, as is indicated in Figs. 1, 2, and 3, the line coupling resistors R2 pass through small holes in the shield structure 3. By this procedure the opposite ends of the resistors are shielded from each other. It is also desirable to provide an additi0na1 shield structure 6 to prevent electrostatic coupling between the input circuits and terminals of the two transmission lines.

In order that a high maximum. voltage ratio may be secured between the two lines T1 and T2 it is important that the contact member I actually be capable of reaching the ends of the inductance L2, so that the minimum inductance in either of the extreme positions of the slider may be negligible. Where a slider is employed as a contact member, as in Figs. 2 and 3, the minimum inductance may further be made low by bringing both ends of L2 as close to the grounded end of the slider as possible. It is important that the impedance from the sliding contact to ground be negligible, since this impedance is common to both sides of the attenuator. Preferably the slider l is of spring steel plated with some material providing low contact resistance between it and the loop L2. 'The slider may be mounted on a rotatable shaft 1 having one end journaled in a wall of shield 3 and havin its other end supported by a bearing 8, and carrying a control knob 9. Good contact between the slider and. the bearing 8 (which is rounded to the shield 3) may be secured by means of a spring washer or a flexible connector (not shown). If these precautions are observed, and proper shielding provided, it is possible to secure voltage raties between the two lines of the order of 20 to l or better.

It is an important feature of the invention that, because the inductance elements L1 and L2 are loosely coupled magnetically, and because the coils are electrostatically shielded from each other, and moreover because the load on L2 is substantially constant, regardless of the adjustment of the contact member, there is no substantial reaction on L1 regardless of load conditions, or of contact member adjustment.

The particular embodiment illustrated in Figs. 2 and 3 employed a single-turn secondary inductance L2 of approximately two inches in diameter. The circular opening 2 in the forward wall of the shield structure was approximately one inch in diameter. The various resistors had the following values: R1=200 ohms, R2=1,00O ohms, R3=100 ohms. The apparatus operated in the range 100 to 200 megacycles. The slider I was adjustable over a 180 arc.

It has been found that the size of the opening 2 determines the magnitude of the maximum voltage obtainable and to some extent the shape of the attenuation curve. The shape of the attenuation curve can also be varied by changing the shape of the loop which forms the inductance L2.

A typical attenuation curve is illustrated in Fig. 4. The curves are plotted in polar coordinates, and consequently the radii drawn from the origin correspond in position to the adjustment of the slider l as viewed in Fig. 3. If the slider is rotated from the right-hand horizontal position, through the angle the voltages applied to the transmission lines T1 and T2 are 61 and c2, respectively. If the slider is rotated through the angle the relative magnitudes of the voltages are reversed.

It has been found that the shape of the attenuation curve can be varied considerably by grounding the center point of the shunt resistor R1, and by varying its resistance value. To obtain this effect the switch S of Fig. 1 is closed. If a relatively low value of R1 is selected (i. e. low with respect to the impedance of Le), the load on the one-turn loop varies over a two-to-one range between the condition where the slider is in the middle of the loop and that where it is at one end. When the slider is at one end, the load on the loop L2 is R1/2, whereas when the slider is in its center position the load is R1. Thus the volt age across the whole of L2 is lower when the slider is at the ends than when it is at the middle. This has the effect of raising the voltage delivered to each line at the middle position, relative to the maximum delivered to the line. Thus, in Fig. 1, when the slider is in the 90 position the voltage delivered to each line is not a great deal less than the maximum voltage delivered in the 0 and 180 positions. An attenuation curve resulting from the use of a relatively low value of R1 is illustrated in Fig. 5.

If R1 is relatively high in impedance, compared to the impedance of L2, the shunt resistor has no substantial effect regardless of the slider position. Fig. 6 is a typical attenuation characteristic which may be secured when R1 is high compared to the impedance of L2.

The device illustrated in Figs. 1 to 3 has been described particularly with reference to its use as a combined voltage divider and attenuator. How ever, it will be evident that the device can also be used solely as an attenuator operating between a source E and a single load circuit, say the line T1. In such use, the resistors R2 and R3, and, of course, the line T2, could be dispensed with. The resistor R1 might, or might not, be retained, depending upon the shape of the attenuation curve desired.

The invention is, of course, capable of various embodiments and modifications such as fall within the scope of the appended claims.

I claim:

1. The combination with a high frequency signal source and a pair of load circuits to be supplied therefrom, said load circuits having a commo n terminal, of a signal divider and attenuator including a first inductor connected to said source, a second inductor having only a portion thereof disposed so as to be magnetically coupled to said first inductor, shielding means for substantially eliminating electrostatic coupling between said inductors, means including an impedance network connecting the respective ends of said second inductor to the respective load circuits, an adjustable contact engaging said second inductor, and means connecting said contact to said common terminal of said load circuits.

2. The combination with a high frequency signal source and a pair of load circuits to be supplied therefrom, of a signal divider and attenuator including a first inductor connected to said source, a second inductor in the form of a single-turn loop connected to said load circuits and having only an intermediate portion magnetically coupled to said first inductor, shielding means for substantially eliminating electrostatic coupling between said inductors, and means including an adjustable contact engaging said second inductor for varying the magnitudes of the voltages applied to said load circuits.

3. The combination with a high frequency signal source and a pair of load circuits to be supplied therefrom, of a signal divider and attenuator including a first inductor connected to said source. a second inductor having only a portion thereof magnetically coupled to said first inductor, an impedance network including series and shunt resistors connecting said second inductor to said load circuits, shielding means for substantially eliminating electrostatic coupling between said inductors, an adjustable contact engaging said seccnd inductor and movable over substantially the entire length thereof, and means connecting said contact to a common terminal of said load circuits.

4. The combination with a high frequency signal source and a pair of load circuits to be supplied therefrom, of a signal divider and attenuator including a first inductor connected to said source, a second inductor having only a portion thereof magnetically coupled to said first inductor, a resistor connected in shunt relation With said second inductor, a pair of series resistors connecting the ends of said second inductor to the respective load circuits, shielding means for substantially eliminating electrostatic coupling between said inductors, an adjustable contact engaging said second inductor and movable over substantially the entire length thereof, and means connecting said contact to a common terminal of said load circuits.

5. The combination with a high frequency si nal source and a pair of load circuits to be supplied therefrom, of a signal divider and attenuator including a first inductor connected to said source, a second inductor in the form of a singleturn loop having only an intermediate portion thereof magnetically coupled to said first inductor, an impedance network including series and shunt resistors connecting said second inductor to said load circuits, shieldin means for substantially eliminating electrostatic coupling between said inductors, an adjustable contact engaging said second inductor and movable over substantially the entire length thereof, and means connecting said contact to a common terminal of said load circuits.

6. The combination with a high frequency signal source and a pair of load circuits to be supplied therefrom, of a signal divider and attenuator including a first inductor connected to said source, a second inductor in the form of a singleturn loop having only an intermediate portion thereof magnetically coupled to said first inductor, a resistor connected in shunt relation with said second inductor, a pair of series resistors connecting the ends of said second inductor to the respective load circuits, shielding means for substantially eliminating electrostatic coupling between said inductors, an adjustable contact engaging said second inductor and movable over substantially the entire length thereof, and means connecting said contact to a common terminal of said load circuits.

7. The combination with a high frequency signal source and a pair of load circuits to be supplied therefrom, of a signal divider and attenuator including a first inductor connected to said source, a second inductor having only a portion thereof magnetically coupled to said first inductor, a resistor connected in shunt relation with said second inductor, a pair of series resistors connecting the ends of said second inductor to the respective load circuits, shielding means for substantially eliminating electrostatic coupling between 'said inductors, an adjustable contact engaging said second inductor and movable over substantially the entire length thereof, means connecting said contact to a common terminal of said load circuits, and means operable at will to connect the mid-point of said shunt resistor to said common terminal.

8. In a high frequency attenuator, an inductor in the form of a single-turn loop, shielding means for said inductor having an opening adjacent an intermediate portion of said inductor, a contact slider engaging saidinductor, and another inductor on the opposite side of said shielding means and having a portion adjacent said opening, whereby only the said portion of the firstnamed inductor is magnetically coupled to said last-named inductor.

9. In a high frequency attenuator, an inductor in the form of a single-turn loop, a shielding enclosure about said inductor, said enclosure having an opening adjacent an intermediate portion of said induct/on a contact arm pivoted adjacent the extremities of said inductor and engaging the same, and another inductor outside said enclosure and having a portion adjacent said opening,

whereby only the said portion of the first-named inductor is magnetically coupled to said lastnamed inductor.

. 10. The combination with a high frequency sig nal source and a pair of load circuits to be sup plied therefrom, of a signal divider and attenuator including an inductor in the form of a single-turn loop, a resistor connected between the extremities of said inductor, a shielding enclosure about said inductor and said resistor, said enclosure having an opening adjacent an intermediate portion of said inductor, another inductor outside said enclosure and having a portion adjacent said opening, whereby only the said portion of the first-named inductor is magnetically coupled to said last-named inductor, means connecting the last-named inductor to said signal source, a pair of series resistors connecting the extremities of g the first-named inductor to said load circuits, an adjustable contact engaging said first-named inductor, and means connecting said contact to a common terminal of said load circuits.

RICHARD G. CLAPP. 

